The invention relates to a push-pull amplifier comprising a first and a second transistor of the same conductivity type, whose collector-emitter paths are included in series between power-supply terminals, the emitter electrode of the first transistor being connected to the collector electrode of the second transistor and to an output terminal, the base electrode of the first transistor being connected to an input terminal, and means being provided for driving the second transistor as a function of the drive of the first transistor.
Modern integrated wide-band amplifiers preferably use transistors of the same conductivity type for the transistors in the push-pull output stage, because in this case, in contradistinction to a so-called "complementary" output stage, satisfactory high-frequency characteristics and high gain-factors are attainable. Such a circuit arrangement is described in "Electronic Letters" Vol. 10, No. 15 to 25 July 1974 on pages 317, 318 and 319.
In order to enable the second transistor in this arrangement to be driven in a suitable manner, the collector circuit of the first transistor includes a diode in the forward direction. The voltage across this diode, which is a measure of the current through the first transistor is converted, via a third transistor which operates in common-base arrangement and which is of a conductivity type opposite to that of the output transistors, into a corresponding current of opposite phase through a resistor in parallel with the base-emitter junction of the second transistor. As a result of this the current through the second transistor will be in phase opposition to that through the first transistor. Moreover, in the case of sufficiently small currents through the third transistor, current is sustained in the output transistors, which is favorable for a satisfactory take-over behavior. However, the transfer is not satisfactorily linear because of the asymmetry of the arrangement, the finite currents through the third transistor, and the drive of the second transistor via a second resistor. This resistor and the deviating conductivity type of the third transistor also give rise to temperature dependence of the bias currents.